Of whom Daniel was first: this was Belshazzar's promise to Daniel, he should be the third ruler in the kingdom, Daniel 5:7,16,29; the first was general of the army, the second president of the palace, the third of the land and provinces.
Related Commentaries of Daniel 6:2
Daniel 6:2
2 And over these three presidents; of whom Daniel was first: that the princes might give accounts unto them, and the king should have no damage.